Glycosides is the collective term for an extensive group of vegetable materials and synthetic compounds which are cleaved by boiling with water or dilute acids or under the action of glycosidases into one or more carbohydrates (mono- or oligosaccharides) and other compounds. The sugars are bonded by an oxygen atom in a glycosidic bond (generally an ether bond) to a hemiacetal carbon atom to give the full acetal. (Römpp Lexikon Chemie—Version 2.0, Stuttgart/New York: Georg Thieme Verlag 1999).
The monomer of the glycosides is usually a monosaccharide, for example, glucose (most frequently) or galactose, mannose, fructose, inter alia, hexoses or the pentoses arabinose, xylose, ribose; additionally, sugars also occur which are found exclusively in glycosides, for example, digitalose, cymarose, and etc. When the carbohydrate radical of a glycoside is a glucose, the derivative is known as a glucoside; analogously, the fructosides are glycosides with fructose as the sugar component, galactosides are glycosides with galactose as the sugar component. (Römpp Lexikon Chemie—Version 2.0, Stuttgart/New York: Georg Tneme Verlag 1999).
Methylglycosides are starting materials for the synthesis of alkylglycosides. These are full acetals which are prepared from sugar hemiacetals. Analogously, the corrpnding ketals are formed from the nonreducing sugars (lit.: A. T. J. W. de Goede, F. van Rantwijk, H. Van Bekkum, Starch/Starke 47 (1995) 233-237). The ketals are an extremely important substance class for the cosmetics industry. The most important compounds by far (possibly after further modifications) are typically used as nonionic surfactants with good foam properties, as emulsifiers or as thickeners. The advantages of these compounds are their low mucosa irritation potential and their biodegradability.
The classical synthesis of alkylglucosides is effected by reaction of sugars or carbohydrates with alcohols in the presence of an acidic catalyst (Fischer glycosidation; lit.: A. F. Bochkov, G. E. Zaikov: The Chemistry of the O-Glycosidic Bond, Pergamon Press, Oxford, 1979, 210 pp.). In order to enable a reaction of the mutually immiscible reactants, the mixture is kept at a high temperature for a long time and the water which forms is removed. Even the simple hydrolysis of a carbohydrate entails relatively drastic conditions resulting from use of 1 M sulfuric acid and 100° C. for several hours (this is the case for hexose-containing polysaccharides; lit.: Frieder W. Lichtentaler, in: Ullmann's Encyclopedia of Industrial Chemistry, “Carbohydrates”). In the course of this, however, partial decomposition of the sugars cannot be prevented. Generally, the product composition depends greatly upon the selected catalyst. In a two-stage synthesis, it is initially possible to prepare a glycoside having a short allyl chain which is subsequently exchanged for another alkyl radical by transacetalization in the presence of an acid.
Typical catalysts for the alkylation of glycosides are sulfuric acid and p-toluenesulfonic acid, as described, for example in U.S. Pat. Nos. 3,772,269 and 3,375,243. Such strong acids cause intense coloration of the product, which necessitates workup of the product. In addition, the use of mixed salts of a strong organic acid and a weak organic base is known (see, for instance, U.S. Pat. No. 5,432,269), but the products prepared in this way are also strongly colored and can contain free organic bases. Salts ofpolybasic carboxylic acids (see, for example, U.S. Pat. No. 4,898,934) and hydroxycarboxylic acids (see, for example, U.S. Pat. No. 4,465,828) are also used as catalysts. However, when the reaction mixture is concentrated, a colored product is unavoidably obtained as a result of the thermal stress in the presence of a concentrating acid. Therefore, a step for decolorization of the product (for example, with H2O2) has to follow in all cases.
The simplest and cheapest glycoside is methylglucoside (see formula II in scheme 1), whose preparation has been known for some time: see Römpp Chemie Lexikon, heading: α-methylglucoside: Preparation by the action of methanol on glucose in the presence of HCl or a cation exchanger. β-Methylglucoside is additionally formed.

This synthesis is also described in numerous patents, see for example, U.S. Pat. No. 2,276,621 (publication date Mar. 17, 1942). The reaction is carried out in methanol as the alcoholic component and simultaneously the solvent, in the presence of a carbohydrate, for example starch. The catalysts used are inorganic acids, for example, sulfuric acid. The carbohydrate hydrolyzes to give glucose and methylglucoside is obtained in the presence of the catalyst.
JP-A-06-092984 describes the use of an immobilized catalyst system based on a cation exchanger resin for the preparation of methylglucoside, which is said to enable a preparation of only slightly colored products. Analogously thereto, the use of Amberlyst 15 is described in DE-A-3 611 035.
FR-1 114 382 describes the reaction of potato starch with methanol in the presence of HCl. A further reaction entails the addition of dilute sulfuric acid and subsequent decolorization.
One method for preparing alkylglycosides directly from oligo- or polyglycosides is, according to the prior art, possible only in a laborious process by hydrolytic cleavage (enzymatically, for example, with glycosidases, or chemically by acid catalysis) in the presence of a catalyst in alcohols.
When the above-mentioned customary methods are used, a multitude of by-products are obtained, and also the products tend to be intensely colored (mainly unreacted sugars), therefore further workup of the product becomes necessary.
The preparation of alkypolyglycosides is described, for example, in EP-B-0 970 097. For the formation of the full acetal, a monosaccharide is reacted at high temperature and standard pressure with an excess of alcohol in the presence of a mixture of sulfuric acid and sodium hydroxide or sodium carbonate as a catalyst. Even though neutralization of the reaction mixture by suitable selection of the catalyst mixture is not normally necessary, an addition of bases may be necessary in this case too. The alcohols used may be any mono- or polyhydric primary or secondary alcohols. Enzymatic glycosidation is also known (JP-A-9087294, EP-A-725 144, JP-A-806 769 0, JP-A-708 799 2).
The reaction of very different sugars, for example, maltose, and even of polymeric nature, is also known. The acid catalyzed reaction of malto-oligosaccharides with alcohols or thiols is described in U.S. Patent Publication No. 2002/099185.
However, all of the prior art preparation processes are time-consuming batch syntheses of the corresponding alkylglycosides in which, owing to the harsh conditions to which the reactants are exposed over a long time, numerous by-products, which cause intensive coloration of the product, can be obtained.
There is still a need to provide a method for the simple and inexpensive synthesis of alkylglycosides, in which these products can be prepared in high space-time yields, continuously or batchwise, by direct reaction of sugars, oligo- or polyglycosides, as far as possible, without additions of catalysts or activators, in such a way that a further workup, for example with bleaches, is not required.